The present invention relates to a method of and apparatus for applying liquid coating material onto a moving web of paper, and in particular to a coating method and apparatus of the fountain applicator type.
Coating a web of paper is generally effected by the application of a liquid coating material onto a moving web. The coating material may be comprised of a solid constituent suspended in a liquid carrier. The quality of the coating applied onto the paper web depends upon a number of factors, an important one of which being how the material is applied. The application of the coating material should preferably result in a coating that is continuous and uniform across the web
One method previously used to coat paper webs was to feed liquid coating material to applicator rolls that applied the material directly onto the moving web. While the use of applicator rolls yields a fairly uniform coating across the web, as web speeds increase there often occurs a film split pattern in the coating applied onto the web, i.e., cross-direction variations in the weight of the coating on the web. This technique therefore does not lend itself to coating webs at high speeds. Direct application by rolls also creates forces in the roll/web nip that imbed or force coating material into the web instead of covering the outer surface of the web to enhance smoothness.
In an attempt to avoid these and other problems, the art developed a coating process in which the liquid coating material was jetted in a free standing curtain of coating liquid directly onto the moving web with a fountain applicator. While fountain applicators overcome many of the limitations of roll applicators, in their use, skip coating often occurs. Skip coating is caused by air entrained in the coating liquid being contacted against the paper web and preventing the coating liquid from uniformly contacting and being uniformly applied onto the web surface. To decrease the severity of the skip coating problem, fountain applicators customarily include coating/air separation equipment to remove air from coating liquid prior to delivery of the coating liquid to the applicator, but the equipment is not 100% effective and some air remains entrained in the coating liquid jetted against the web surface and causes skip coating.
An object of the present invention is to provide an improved fountain applicator for applying liquid coating material onto a paper web, in which the resulting coating on the web is substantially skip free.
Another object is to provide such a fountain applicator, in which a sheet of coating liquid is flowed along a curved surface substantially immediately prior to being impinged against the web, to subject the sheet to centrifugal force to cause air entrained in the coating liquid to move away from the curved surface.
A further object is to provide such a fountain applicator, in which the sheet of coating liquid, after leaving the curved surface, is directed toward the web in a free standing jet curtain of coating liquid that is impinged against the web, to contact the web surface primarily with the side of the jet curtain of coating liquid that was toward the curved surface and is relatively free of entrained air, to decrease the occurrence of skip coating on the web surface.
Yet another object is to provide such a fountain applicator in a paper coating system that includes a downstream doctor for metering and leveling on the web surface an excess coating layer applied onto the web surface by the applicator.
In accordance with the present invention, an applicator for applying coating liquid onto a surface of a moving web comprises an elongate concave curved surface that is positionable proximate to, transversely of and spaced from the web; and means for forming an elongate sheet of coating liquid, for flowing the sheet along the curved surface, and for then projecting the sheet in a free standing jet curtain of coating liquid against and across the surface of the web. The coating liquid sheet, upon being flowed along the curved surface, is subjected to centrifugal force to cause air entrained in the coating liquid sheet to move away from one side of the sheet that is toward the curved surface, so that the one side is then relatively free of entrained air. The free standing jet curtain of coating liquid is directed against the web to contact the web surface primarily with the one relatively air-free side of the coating liquid sheet to decrease the occurrence of skip coating on the web surface.
The applicator applies the coating in excess onto the web surface, and also included are downstream doctor means for metering and leveling the excess coating layer to a desired coat weight. The doctor means may comprise a single metering device. Alternatively, the doctor means may comprise a first metering device for partially metering and leveling the coating layer, followed by a second and final metering device for metering and leveling the coating to the desired final coat weight.
The invention also contemplates a method of applying a coating liquid onto a surface of a moving web, which comprises the step of flowing a sheet of coating liquid along an elongate concave curved surface that is proximate to, extends transversely of and is spaced from the web, to subject the coating liquid sheet to centrifugal force to cause air entrained in the coating liquid to move away from one side of the sheet that is toward the curved surface, so that the one side of the sheet is relatively free of entrained air. Also, included is the step of directing the sheet of coating liquid, after it has been flowed along the curved surface, toward the web in a free standing jet curtain of coating liquid, to contact the web surface primarily with the one relatively air-free side of the coating liquid sheet to decrease the occurrence of skip coating on the web surface.
The coating liquid is applied is excess onto the web surface, and the method also includes the step of doctoring the excess coating layer on the web to a final coat weight. The doctoring step may comprise doctoring the excess coating layer with a single metering device. Alternatively, the doctoring step may comprise metering and leveling the coating layer with an initial metering device, followed by metering and leveling the coating layer to a desired final coat weight with a second metering device.